1. Field of the Invention
This invention relates to radiation sensitive compositions. The compositions have particular utility in electrophoretic deposition processes including the manufacture of electronic printed circuit boards and other electronic packaging substrates. More specifically, the amphoteric compositions of the invention contains a polymer that can be selectively treated to render the composition either anodic or cathodic as desired and imaged and stripped in either acid or base chemistries.
2. Background Art
Radiation sensitive compositions include photoresists which are photosensitive coatings capable of transferring an image onto a conductive substrate such as a printed circuit board or a lithographic plate. The photoresist may be of the liquid type which is typically applied by dip-coating, roller-coating, spin-coating or by screen printing. These resists can be negative-acting or positive-acting. In the case of negative-acting resists, after the film is deposited on a surface and the solvent is removed as by heating, the film is selectively exposed, typically through a photomask, to a source of activating energy such as ultraviolet light. The photomask has areas that are opaque and other areas that are transparent to the ultraviolet light. The pattern on the photomask formed by opaque and transparent areas defines the pattern, such as a circuit, to be transferred to the substrate.
Exposed portions of a negative-acting photoresist become less soluble in a developing solution than unexposed areas as the result of a photochemical reaction, for example, between a photoinitiator and an ethylenically unsaturated resin of the resist composition. This difference in solubility allows for the selective removal of the photoresist and the transfer of the photomask image to the substrate. In the case of positive-acting photoresists, exposure to activating radiation will cause the coating to become more soluble in a developing solution than the unexposed areas. Such photoresists are generally well known and are typically formulated from materials such as vinyl cinnamate copolymers, such as disclosed in U.S. Pat. Nos. 2,737,297; 2,739,892; and 2,801,233; benzal acetophenones such as disclosed in U.S. Pat. Nos. 2,716,097 and 2,716,102; quarternary salts such as disclosed in U.S. Patent No. 2,908,667: azide polymers such as disclosed in U.S. Pat. No. 2,948,610; and naphthaquinone diazide esters and amides such as disclosed in U.S. Pat. Nos. 3,634,082; 4,148,654 and 4,439,516, all incorporated herein by reference. The imaged photoresist may act as a protective coating for subtractively etching the metal substrate, known as print-and-etch, or allow the exposed bare metal surface to be further built up in thickness by electroplating methods, known as print-and-plate. The resist may then be selectively stripped. For print-and-plate processes, the exposed metal on the substrate may be etched to form the desired pattern or circuit on the substrate surface. The historical background, types and operation of conventional photoresists, are generally described in Photoresist Materials and Processes, W. DeForest, McGraw-Hill, 1975.
Another type of photoresist is commonly referred to as dry film. A dry film resist is a multilayer film in which a solid photosensitive film is sandwiched between a polyethylene film and polyester cover sheet. Dry film resists are described, for example, in U.S. Pat. Nos. 3,469,982, 4,378,264 and 4,343,885. As in liquid resists, they can be either negative or positive acting. Dry film resists are typically laminated onto a printed circuit board by application with hot rollers, after the bottom polyethylene film is peeled from the remaining photoresist film and polyester cover sheet. The transparent cover sheet usually remains on the board to provide mechanical support and to protect the photoresist. After exposure to activating radiation, the cover sheet is removed prior to development.
Electrophoretic deposition of certain photoresist coatings has been reported. Electrophoretic deposition involves a process of electrophoresis which is the motion of charged particles through a liquid medium under the influence of an applied electrical field. The deposition is conducted in a cell with the conductive substrate to be coated serving as one of the electrodes. Deposition of a positively charged material or mixture of materials onto a negatively charged cathode is referred to as cataphoresis while deposition of a negatively charged material or mixture of materials onto a positively charged anode is referred to as anaphoresis.
U.S. Pat. No. 3,738,835 describes the use of anaphoresis to deposit a photosensitive composition of polychloroprene polymer and a photosensitizer such as 4,4-bis(dimethylamino)-benzophenone in a mixed organic solvent. U.S. Pat. Nos. 3,954,587; 4,029,561; 4,035,273; 4,035,274; 4,039,414; 4,066,523; 4,070,258; and 4,166,017 also describe coatings that are applied electrophoretically and exposed to activating radiation.
U.S. Pat. No. 4,592,816 describes a negative-acting electrophoretic photoresist that contains at least one polymer that incorporates an ionizable group. The described photoresist compositions incorporate the use of ethylenic unsaturated monomers to crosslink a film layer. For the cataphoretic photoresists described, the developer and stripper solutions required to remove desired areas of the photoresist are acidic in nature. The described anaphoretic photoresists require the use of base to develop an image. This generally exemplifies the art in that known cataphoretic negative working photoresists are imaged and removed only in acid solutions such as lactic acid; and, in like manner, known anaphoretic negative working photoresists are imaged and removed only in base solutions. Such restriction of development and stripping chemistries can impose significant limitations in some processes.
It thus would be desirable to have a cataphoretic photoresist that could be developed and/or stripped with a base solution, and to have an anaphoretic photoresist that could be developed and/or stripped with an acidic solution. It also would be desirable to have a single radiation sensitive composition that could be electrodeposited on a conductive substrate either cataphoretically or anaphoretically. It would be further desirable if such a composition could be formulated as a liquid coating composition or used to form a dry film resist.